1. Field of the Invention
The present invention relates generally to a signal transmission/reception system and method in a mobile communication system. More particularly, the present invention relates to a system and method to transmit/receive signals in a Time-Division Duplex (TDD) mobile communication system using multi-hop scheme (hereinafter referred to as a “multi-hop TDD mobile communication system”).
2. Description of the Related Art
Intensive research in a next generation communication system is being conducted to provide Mobile Stations (MSs) with high-speed services having various high Quality-of-Service (QoS) classes. Multi-hop scheme is a conventional transmission/reception scheme, the use of which is positively considered as a next generation communication system. With reference to FIG. 1, a description will now be made of a configuration of a multi-hop TDD mobile communication system.
FIG. 1 is a diagram illustrating a configuration of a general multi-hop TDD mobile communication system.
Referring to FIG. 1, the multi-hop TDD mobile communication system includes an Internet backbone 111, a mobile communication network 113, multiple Base Stations (BSs) of BS#1 115-1 and BS#2 115-2, multiple Relay Stations (RSs) of RS#1 117-1 and RS#2 117-2, and multiple MSs of MS#1 119-1, MS#2 119-2, MS#3 119-3 and MS#4 119-4.
The RSs 117-1 and 117-2 are installed for throughput enhancement or service coverage extension. The internet backbone 111 is connected to the mobile communication network 113, and the mobile communication network 113 performs communication with the BSs 115-1 and 115-2. The BS#1 115-1 performs communication with the RSs 117-1 and 117-2. The BS#1 115-1 is a parent BS for the RSs 117-1 and 117-2, and the RSs 117-1 and 117-2 are children RSs of the BS#1 115-1.
The RS#1 117-1 performs communication with the MSs 119-1, 119-2 and 119-3. The RS#1 117-1 is a parent RS of the MSs 119-1, 119-2 and 119-3, and the MSs 119-1, 119-2 and 119-3 are children MSs of the RS#1 117-1. The MS#4 119-4 performs direct communication with a BS, i.e. BS#1 115-1, without any RS. A MS that performs direct communication with a BS without any RS in this way will referred to as a ‘legacy MS’. Therefore, the MS#4 119-4 is a legacy MS of the BS#1 115-1.
For a downlink (DL) interval, the RSs 117-1 and 117-2 receive the signals that the BS#1 115-1 has transmitted to the targets of the MSs 119-1, 119-2 and 119-3, and retransmit the received signals to the MSs 119-1, 119-2 and 119-3 over an RS-MS link having a channel quality higher than the channel quality between the BS#1 115-1 and the MSs 119-1, 119-2 and 119-3.
For an uplink interval, the RSs 117-1 and 117-2 receive the signals transmitted from the MSs 119-1, 119-2 and 119-3, and retransmit the received signals to the BS#1 115-1. The signal retransmission operation performed for the downlink interval and uplink interval can be performed according to an Amplify-and-Forward (AF) mode, or can be performed according to a Decode-and-Forward (DF) mode. A description of the AF mode and DF mode will be made below.
In the AF mode, an RS simply amplifies a received signal and then forwards it to its corresponding children MSs. Therefore, in the AF mode, the amplified signal, whose additive white noise is also amplified, is retransmitted from the RS to the MSs.
In the DF mode, an RS decodes a signal received from a BS, performs re-encoding and re-scheduling on the decoded signal where applicable, and then retransmits it to corresponding MSs. As a result, the AF mode results in a relatively simple RS structure, while the DF mode is preferable in terms of the reliability and efficiency of the relay transmission.
Generally, the multi-hop TDD mobile communication system operates in a half-duplex mode. In the half-duplex mode, the multi-hop TDD mobile communication system transmits a downlink signal and an uplink signal in sequence. It will be assumed herein that a basic transmission unit of the downlink signal is a downlink frame, and a basic transmission unit of the uplink signal is an uplink frame.
First, a description will be made of an operation of transmitting the downlink frame.
In a first step, for a time that an RS transmits no downlink frame, a BS transmits a downlink frame to a child RS and a legacy MS. In a second step, the RS transmits a downlink frame, which contains information for those children MSs of the said RS and received from the said first downlink frame transmitted by the said BS in the first step, to its children MS.
Second, a description will be made of an operation of transmitting the uplink frame.
In a first step, for a time that an RS transmits no uplink frame, a child MS transmits an uplink frame to the RS, and a legacy MS transmits an uplink frame to a BS. In a second step, the RS transmits a uplink frame, which contains information received from its children MS during the said uplink frame transmitted by the said children MSs in the first step, to the RS's parent BS.
As described above, the half-duplex mode is an operation mode based on the assumption that the RS cannot simultaneously perform a signal transmission operation and a signal reception operation. In addition, the half-duplex mode is an operation mode based not only on a near-field effect of antennas, but also on compatibility with a frequency reuse factor=1.
When the multi-hop TDD mobile communication system operates in the half-duplex mode, interference occurrence between a BS and an RS is prevented in a downlink frame interval, and interference occurrence between an RS and an MS is prevented in an uplink frame interval. However, when the multi-hop TDD mobile communication system uses the half-duplex mode, a data rate becomes ½, causing a limitation in the data rate.
The wireless channel environment of the mobile communication system, unlike a wired channel environment, may suffer from an information loss caused by inevitable errors due to various factors such as multi-path interference, shadowing, propagation loss, time-varying noise, interference, fading, etc. The information loss causes considerable distortion to an actual transmission signal, reducing the entire performance of the mobile communication system.
Therefore, the next generation communication system positively considers using a diversity scheme to avoid instability of communication due to the fading. The diversity scheme can be roughly classified into a time diversity scheme, frequency diversity scheme and an antenna diversity scheme, or space diversity scheme.
The antenna diversity scheme, a multiple antenna technique, can be classified into a reception antenna diversity scheme using multiple reception antennas, a transmission antenna diversity scheme using multiple transmission antennas, and a Multiple Input Multiple Output (MIMO) scheme using multiple reception antennas and multiple transmission antennas. The MIMO scheme is a kind of a Space-Time Coding (STC) scheme, and the STC scheme transmits signals encoded by a predetermined coding scheme, using multiple transmission antennas, thereby extending a time-domain coding scheme to a space-domain coding scheme, and thus achieving a lower error rate.
As described above, the use of the MIMO scheme may reduce the information loss, thus facilitating efficient high-speed, high-capacity data transmission. However, the current multi-hop TDD mobile communication system has provided no detailed scheme for the use of the MIMO scheme.